Centralizers are often employed in oilfield and related industries where controlled positioning of a device within a well may be of importance. A well is any boring through the earth's surface that is designed to find and acquire liquids and/or gases. Wells for acquiring oil are termed “oil wells.” A well that is designed to produce mainly gas is called a “gas well.” Typically, wells are created by drilling a bore, typically 5 inches to 40 inches (12 cm to 1 meter) in diameter, into the earth with a drilling rig that rotates a drill string with an attached bit. After the hole is drilled, sections of steel pipe, commonly referred to as a “casing” and which are slightly smaller in diameter than the borehole, are dropped “downhole” into the bore for obtaining the sought after liquid or gas.
The difference in diameter of the wellbore and the casing creates an annular space. When completing oil and gas wells, it is important to seal the annular space with cement. This cement is pumped in, often flushing out drilling mud, and allowed to harden to seal the well. To properly seal the well, the casing should be positioned so that it is in the middle or center of the annular space. The casing and cement provides structural integrity to the newly drilled wellbore in addition to isolating potentially dangerous high pressure zones from each other and from the surface. Thus, centralizing a casing inside the annular space is critical to achieve a reliable seal and, thus, good zonal isolation. With the advent of deeper wells and horizontal drilling, centralizing the casing has become more important and more difficult to accomplish.
Additionally, in the case of a hydrocarbon well, there may arise the need to deliver a downhole tool several thousand feet down into the well for performance of an operation. In performing the operation, it may be preferable that the tool arrive at the operation site in a circumferentially centered manner (with respect to the diameter of the well). Therefore, a centralizer may be associated with the downhole tool in order to ensure its circumferentially-centered delivery to the operation site. This may be especially beneficial where the well is of a horizontal or other configuration presenting a challenge to unaided centralization.
Centralization of one or more components of a well may be advantageous for a host of other different types of operations. In many operations, the vertical alignment of multiple separately delivered downhole tools may be beneficial. In this manner, centralization of such tools at an operation site provides a known orientation or positioning of the tools relative to one another. This known orientation may be taken advantage of where the tools are to interact during the course of the operation, for example, where one downhole tool may be employed to grab onto and fish out another. Additionally, a host of other operations may benefit from the circumferentially-centered positioning of a single downhole tool. Such operations may relate to drilling performance, oil well construction, and the collection of logging information, to name a few.
A traditional method to centralize a casing is to attach centralizers to the casing prior to its insertion into the annular space. Traditional centralizers are commonly secured at intervals along a casing string to radially offset the casing string from the wall of a borehole in which the casing string is subsequently positioned. Most traditional centralizers have wings or bows that exert force against the inside of the wellbore to keep the casing somewhat centralized. The centralizers generally include evenly-spaced arms or ribs that project radially outwardly from the casing string to provide the desired offset. The radially disposed arms or ribs are biased outwardly from a mandrel or other supporting body in order to contact sides of the well wall and, thus, centrally positioning the supporting body. Centralizers ideally center the casing string within the borehole to provide a generally continuous annulus between the casing string and the interior wall of the borehole. This positioning of the casing string within a borehole promotes uniform and continuous distribution of cement slurry around the casing string during the subsequent step of cementing the casing string in a portion of the borehole. Uniform cement slurry distribution results in a cement liner that reinforces the casing string, isolates the casing from corrosive formation fluids, prevents unwanted fluid flow between penetrated geologic formations, and provides axial strength. Unfortunately, these centralizers increase the profile of the casing, thereby causing increased resistance and potential snagging during casing installation.
A bow-spring centralizer is a common type of centralizer that employs flexible bow-springs as the ribs. Bow-spring centralizers typically include a pair of axially-spaced and generally aligned collars that are coupled by multiple bow-springs. The bow-springs expand outwardly from the axis of the centralizer to engage the borehole sidewall to center a pipe received axially through the generally aligned bores of the collars. Configured in this manner, the bow-springs provide stand-off from the borehole and flex inwardly as they encounter borehole obstructions (such as tight spots or protrusions into the borehole) as the casing string is installed into the borehole. Elasticity allows the bow-springs to spring back to substantially their original shape after passing an obstruction to maintain the desired stand-off between the casing string and the borehole.
Unfortunately, the delivery of a downhole tool through the use of a centralizer is prone to inflict damage at the wall of the well by the radially disposed arms of the centralizer. This is because the centralizer is configured with arms reaching an outer diameter capable of stably supporting itself within wider sections of the well. For example, the centralizer may reach a natural outer diameter of about 13 inches for stable positioning within a 12 inch diameter section of a well. However, the centralizer is generally a passive device with arms of a single size that are biased between the support body and the well wall. Therefore, as the diameter of the well becomes smaller, the described arms (often of a bow-spring configuration) are forced to deform and compress to a smaller diameter as well. For example, the same 12 inch diameter well may become about 3 inches in diameter at some point deeper within the well. This results in a significant amount of compressive force to distribute between the arms and the wall of the narrowing well. That is, as the bowed arms become forced down to a lower profile by the narrowing well wall, more force is exerted on the well wall, thereby potentially resulting in damage to the well wall and/or the centralizer.
The above described exertion of force can become quite extreme depending on the configuration and dimensions of the arms and the extent of the well's narrowing. As a result, such bow-spring arms may prematurely wear out or cause significant damage to the well wall as the centralizer is forced through narrower well sections, or may require excessive amounts of force to push down long laterals. Many of these narrower well sections may have no relation to the actual operation site. Thus, the damage to the well wall and/or centralizer may occur in sections of the well where centralization by the centralizer is unnecessary. Furthermore, due to the forces between the centralizer and the well wall, a significant amount of additional force, for example, through coiled tubing advancement, may be required. This may leave coiled tubing, the centralizer, and even the well itself susceptible to damage from application of such greater forces thereupon. This excessive force may restrict the ability to unstick pipe or liner, cause significant problems and non-productive time, and potentially require using smaller diameter casing or tubing to be used, thereby restricting well output.
As an alternative to the passive centralizers described above, active centralizers such as tractoring mechanisms or other devices capable of interactive or dynamic arm diameter changes may be employed. However, these types of devices are fairly sophisticated and generally require the exercise of operator control over the centralizer's profile throughout the advancement or withdrawal of the device from the well. Thus, such mechanisms are prone to operator error which may lead to well damage from the above described passive centralizer. Furthermore, rather than reliance on the radially extending natural force of a bowing or similar arm, such devices may require the maintenance of power to the arms at all times in order to attain biasing against the well wall with the arms. Therefore, unlike a passive centralizer, the active centralizer may fail to centralize when faced with a loss of power.
Attempts have been made to develop low-profile, deployable centralizers that can be added to the outside of the casing/pipe. These are designed to reduce friction and snagging due to the fact that the supports or bows are retracted until in their final position. The challenge in developing an effective deployable centralizer is to make it as low profile as possible, actuate deployment upon demand, and to overcome de-centralizing force.
Centralizers are usually assembled at a manufacturing facility and then shipped to the well site for installation on a casing string. The centralizers, or subassemblies thereof, may be assembled by welding or by other means such as displacing a bendable and/or deformable tab or coupon into an aperture to restrain movement of the end of a bow-spring relative to a collar. Other centralizers are assembled into their final configuration by riveting the ends of a bow-spring to a pair of spaced apart and opposed collars. The partially or fully assembled centralizers may then be shipped in trucks or by other transportation to the well site.
U.S. Pat. No. 6,871,706 (incorporated herein by reference) discloses a centralizer that requires a step of bending a retaining portion of the collar material into a plurality of aligned openings, each to receive one end of each bow-spring. This requires that the coupling operation be performed in a manufacturing facility using a press. The collars of the prior art centralizer are cut with a large recess adjacent to each set of aligned openings to accommodate passage of the bow-spring that is secured to the interior wall of the collar. The recess substantially decreases the mechanical integrity of the collar due to the removal of a large portion of the collar wall to accommodate the bow-springs. The collars of the casing centralizer disclosed in this patent also require several additional manufacturing steps, including the formation of both internal and external (alternating) upsets in each collar to form the aligned openings for receiving and securing bow-springs, a time-consuming process that further decreases the mechanical integrity of the collar.
U.S. Pat. No. 4,545,436 and Great Britain Patent No. 2242457 (incorporated herein by reference) both disclose casing centralizers having a plurality of bow-springs which are connected at either end to the first and second collars. As described in U.S. Pat. No. 4,545,436, the bow-springs are connected to the collars using rivets or by welding. Conversely, in Great Britain Patent No. 2242457, the bow-springs are connected using nuts and bolts.
Additional centralizers are discussed in U.S. Pat. Nos. 2,654,435; 3,746,092; 4,776,397; 5,379,838; 6,457,519; 7,140,431; 7,775,272; 7,857,063; 8,235,106; 8,360,161; and 9,458,672, all of which are incorporated herein by reference.
Improved centralizers and methods continue to be sought, particularly in view of the limitations of the prior art and the need for better and stronger centralizers. Considerations for the development of new centralizers and new methods of assembling the centralizers include manufacturing costs, shipping costs, the costs associated with installing the centralizers onto pipe strings, and the ease of running the pipe string into the well.